1. Field of the Invention
This invention relates to a mobile radio terminal capable of employing different systems while moving, such as international roaming.
2. Description of the Related Art
A conventional mobile radio terminal, as is well known, classifies frequencies used for communications with a base station into a plurality of lists in accordance with possibilities of use of the frequencies and searches for available frequencies in order of the lists having higher possibilities of use thereof (see, for example, Jpn. Pat. Appln. KOKAI Publication No. 2002-354532).
Incidentally, if the conventional mobile radio terminal is located outside its service area for a long time, it searches for the frequencies on the basis of the lists including all of the frequencies (i.e. full-frequency search) and certainly detects the base station with which it can carries out communications, for example, in a cycle of about four hours.
In this conventional method, however, if the user of the mobile radio terminal moves overseas and attempts carrying out the international roaming within its service area, it takes, at longest, about four hours to detect the base station capable of carrying out communications.
If the mobile radio terminal fails to detect the base station for some reasons (for example, temporary shadowing or the like) by conducting the full-frequency search, the mobile radio terminal cannot detect the base station for more four hours.
To respond to this situation, shortening the cycle of conducting the full-frequency search can be conceived. However, the user may be located outside the service area for a long time before moving to a destination and the battery power may be wasted due by conducting unnecessary full-frequency search.
Thus, the cycle of searching for the base station capable of carrying out communications is fixed in the conventional mobile radio terminal. Therefore, if the mobile radio terminal is located outside the service area of, for example, the international roaming service, for a long time, the mobile radio terminal may be unable to detect the base station capable of carrying out communications for a long time or may waste the battery power.